The present invention relates to a method for the preparation of a salt mixture, composed of potassium cyanate, potassium carbonate and sodium carbonate as a product in piece form. The product can be used for the purpose of hardening steel parts. The method can be carried out by reacting urea with potassium carbonate and admixing sodium carbonate, with recovery of the ammonia formed during the reaction in the form of hexamethylenetetramine by means of formaldehyde.
Salt mixtures containing ions of sodium, potassium, carbonate and cyanate in predetermined quantitative proportions are used in liquid form for the hardening of steel parts according to well known techniques. For the melting and replenishing of these hardening salt baths, the salt mixture has to be prepared in the form of solid pieces or lumps. Powders cannot be used for practical technical reasons.
In order to prepare these shaped pieces of hardening salt, it is known for example that in the first place urea is reacted with potassium carbonate (approximately 2.8 moles of urea with 1 mole of potassium carbonate) to form potassium cyanate, with the liberation of ammonia, carbon dioxide and water. The yield is approximately 65 to 70% based on the amount of urea. The ammonia thus liberated is converted by means of an aqueous formaldehyde solution to hexamethylenetetramine, which can be utilized elsewhere. The approximately 98% potassium cyanate thus prepared is mixed with the quantities of potassium carbonate and sodium carbonate required for the hardening salt mixture, melted at 600.degree. C. and cast into suitable composites.
This known method has the disadvantage that, in the reaction of urea with potassium carbonate, at a temperature of about 400.degree. C. sublimation of urea occurs, which contaminates the solution of hexamethylenetetramine and leads to problems in utilizing the latter. Furthermore, the finished powder mixture has to be melted, as this powder cannot be compacted using the conventional powder presses. Melting, for which temperatures of approximately 600.degree. C. are required, results in material handling problems and the formation of cyanide, which is undesirable.
Moreover, it is known that a mixture of urea, potassium carbonate and sodium carbonate in the mixing ratio necessary for the composition of the hardening salt can be heated up to approximately 600C. Ammonia, carbon dioxide and water are evolved instantly at the onset of the reaction, so that the ammonia must be combusted, as it carries with it excessive contamination by entrained urea and alkali carbonates and hence cannot be usefully employed. The reacted melt must then be cast into composites.
This method has the disadvantage that the yield of potassium cyanate, based on urea, is only about 60% and varies greatly within the individual batches. This varying cyanate content has an adverse effect on the quality of the hardening salt. The melt has the same problems as the above-mentioned method. Moreover, the ammonia can no longer be recycled.